perf tests mmap-basic: Remove unused variable to address clang 15 warning

perf tests mmap-basic: Remove unused variable to address clang 15 warning

A clang 15 build reveal several unused-but-set variables, removing the
'foo' variable in tests/mmap-basic.o object to address one of those
cases.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20220929140514.226807-2-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>